回执
receipt (written acknowledgement of receipt of an item)
悔之无及
too late for regrets (idiom); It is useless to repent after the event.
挥之不去
impossible to get rid of
不费吹灰之力
as easy as blowing off dust; effortless; with ease
呼之即来,挥之即去
to come when called (idiom); ready and compliant; always at sb's beck and call